Why do they use cobalt? For imaging?! I always assumed they used titanium or platinum :/

  Cobalt-chrome is the most commonly used metal for hip implants. Titanium is also used but is a softer metal with similar properties to bone.

with known risks:

Cobalt toxicity — an emerging clinical problem in patients (2011) - https://www.mja.com.au/journal/2011/194/12/cobalt-toxicity-e...

Cobalt-chromium toxicity following revision of total hip replacement (2024) - https://pmc.ncbi.nlm.nih.gov/articles/PMC11115421/

Because the human body is warm and wet and salty and so despite the forces being low when your criteria are "super long wearing" and "non-reactive" you wind up with kind of a shitty list of options.

It's "supposed" to remain in solid form.